Echo what Laurierace said. I managed a farm for a major consignor and all the yearlings were out at night (14hrs). Colts were in individual paddocks, they played too rough. Fillies were turned out in groups up to 10-12 in 20ac fields. They stayed in during the day to stay out of the heat, and prevent sunbleaching.
Personally, I think sales prep is a huge boost to starting horses under saddle. 3-4 months of daily walking puts some good condition on them, and they’re well-prepared to start breaking compared to a less-handled homebred straight out of a field.
Mares, foals, and weanlings lived outside. I tend to roll my eyes at folks who say their TB can’t survive without a stall…aside from stabling at the track, TBs (even VERY expensive ones) live a large part of their lives out in a field and do quite well.